BATCH OIL RETURN SYSTEM AND METHOD

Abstract

A refrigeration system comprises a compressor, condenser, separator, and evaporator. A condenser inlet is connected to an outlet of the compressor. A separator inlet is connected to an outlet of the condenser. An evaporator inlet is connected to a first outlet of the separator. An evaporator outlet is connected to the separator inlet. The refrigeration system includes a refrigerant with an oil. A control unit has instructions for executing a method comprising the following steps, removing the refrigerant within the evaporator and the separator to leave the oil in the evaporator and the separator; and draining the oil from the evaporator and the separator.

Description

CROSS-REFERENCE(S) TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No. 63/481034, filed on Jan. 23, 2023, the disclosure of which is fully incorporated herein expressly by reference.

BACKGROUND

U.S. Pat. No. 5,857,347 and International publication WO 2023/198787 disclose “LVS” for use in refrigeration systems in FIGS. 1 and 2. LVS is a highly efficient separator, separating liquid from vapor using a vortex method. In this disclosure, LVS refers to the system of the evaporator 4, the separator 5, and the interconnecting pipes between the evaporator and separator.

Dry refrigerant vapor leaves the top connection of the separator 5 and the liquids will remain. While the refrigerant liquid is volatile, it boils and leaves the separator as vapor, and other liquids that are not volatile under the conditions in the separator will stay. (Such liquids are oil and water). Refrigeration compressors normally have oil carry over and need to be equipped with oil separators at the compressor outlet.

Industrial refrigeration systems are often equipped with highly efficient oil separators at the high-pressure outlet.

The type of oil separators used in commercial refrigeration systems designed for direct expansion (DX) feed evaporators may have medium or low efficiency oil separators. These oil separators recover the major part of the compressor oil, but the oil carry over to the system is still significant. The oil that migrates then needs to be continuously returned to the compressor, often via the dry inlet. The DX refrigeration system (pipework, heat exchangers etc.) needs to be designed for this.

CO2 is more frequently used as refrigerant for commercial DX refrigeration systems. The small commercial compressors give higher capacity with CO2 than with previously used refrigerants, capacities that may reach into the industrial range.

The commercial systems are less costly than industrial systems and are therefore expected to be more frequently used. However, DX feed evaporators can reduce the capacity of a food freezer compared to one equipped with flooded evaporators. Separator/evaporator feed, pump feed, and traditional gravity feed freezers use flooded evaporators.

SUMMARY

Accordingly, this disclosure relates to the use of separator/evaporator in a refrigeration system and an oil return system and program sequence to return oil to the compressor. By using separator/evaporator feed instead of DX, the freezer capacity as well as system performance and efficiency is increased.

An embodiment is related to a refrigeration system, comprising: a compressor, condenser, separator, and evaporator, wherein; a condenser inlet is connected to an outlet of the compressor; a separator inlet is connected to an outlet of the condenser; an evaporator inlet is connected to a first outlet of the separator; an evaporator outlet is connected to the separator inlet; wherein the refrigeration system includes a refrigerant with an oil; a control unit having instructions stored thereon for executing an oil batch return sequence comprising the following steps, boiling the refrigerant within the evaporator and the separator that leaves the oil in the evaporator and the separator; and draining the oil from the evaporator and/or separator.

In an embodiment, the oil is drained to a compressor suction line or to a vessel connected to the compressor suction line.

In an embodiment, the control unit further has instructions stored thereon for executing the step of: before draining the oil, checking a freezer temperature is above a predetermined setpoint.

In an embodiment, the step of draining the oil includes opening a drain valve while a timer counts down for a predetermined time, and closing the drain valve when the predetermined time expires.

In an embodiment, the control unit further has instructions stored thereon for executing the step of: keeping the drain valve open for duration of the predetermined time as long as an oil batch return allowed signal from a refrigeration sequence is true.

In an embodiment, the drain valve is placed on a line connecting a bottom of the separator to a vessel connected to a suction side of a compressor.

In an embodiment, the control unit further has instructions stored thereon for executing the step of: draining the evaporator and separator of oil via providing a differential pressure between a refrigeration suction side and the separator and evaporator.

In an embodiment, the differential pressure is measured across a control valve, the control valve is placed on a line connecting a top of the separator and a suction side of the compressor.

In an embodiment, the control unit further has instructions stored thereon for executing the steps of: when the differential pressure is above a setpoint opening the control valve; and when the differential pressure is below the setpoint closing the control valve.

In an embodiment, the control unit further has instructions stored thereon for executing the steps of: when the differential pressure is below a setpoint, running the compressor and the control valve controls the differential pressure.

In an embodiment, the control unit further has instructions stored thereon for executing the steps of: measuring a first pressure of the suction side of the compressor; when the first pressure is below a setpoint, adding liquid refrigerant to the separator.

In an embodiment, the control unit further has instructions stored thereon for executing the steps of: measuring a second pressure of the top of the separator; when the second pressure is above a setpoint, stop adding liquid refrigerant to the separator.

In an embodiment, the control unit further has instructions stored thereon for executing a refrigeration sequence comprising the following steps: sending a signal from the refrigeration sequence to the oil batch return sequence to allow the oil batch return sequence to run.

In an embodiment, the control unit further has instructions stored thereon for executing the following step: sending a signal from the refrigeration sequence to the oil batch return sequence to stop the oil batch return sequence from running.

In an embodiment, the control unit further has instructions stored thereon for executing the following step: sending a signal from the oil batch return sequence to the refrigeration sequence to request the refrigeration sequence to start a compressor running.

In an embodiment, the control unit further has instructions stored thereon for executing the following step: after the oil has been drained, boiling off any remaining liquid refrigerant.

In an embodiment, the control unit further has instructions stored thereon for executing the following step: after elapse of the predetermined time, equalizing a pressure difference between a first pressure of the separator/evaporator and a second pressure of a compressor suction side.

In an embodiment, the control unit further has instructions stored thereon for executing the following steps: during equalizing, measuring the pressure difference and a control valve positioned between the separator/evaporator and the compressor suction side is opened to lower the pressure difference until the pressure difference is close to zero.

In an embodiment, a method of draining oil from a refrigeration system which includes a compressor, condenser, separator, and evaporator, a condenser inlet is connected to an outlet of the compressor; a separator inlet is connected to an outlet of the condenser; an evaporator inlet is connected to a first outlet of the separator; an evaporator outlet is connected to the separator inlet; the separator and the evaporator using a refrigerant with an oil, the method comprising: removing the refrigerant within the evaporator and the separator that leaves the oil in the evaporator and the separator; and draining the oil from the evaporator/separator.

In an embodiment, the method further comprises, before draining the oil, checking a freezer temperature is above a predetermined setpoint.

In an embodiment, the step of draining the oil includes opening a drain valve while a timer counts down for a predetermined time, and closing the drain valve when the predetermined time expires.

In an embodiment, the method further comprises, keeping the drain valve open for duration of the predetermined time as long as an oil batch return allowed signal from a refrigeration sequence is true.

In an embodiment, the method further comprises, draining the evaporator and separator of oil via providing a differential pressure between a refrigeration suction side and the separator and evaporator.

In an embodiment, the method further comprises, measuring the differential pressure; when the differential pressure is above a setpoint opening a control valve; and when the differential pressure is below the setpoint closing the control valve, wherein the control valve is placed on a line connecting a top of the separator and a suction side of the compressor.

In an embodiment, the method further comprises, when the differential pressure is below a setpoint, running the compressor and the control valve controls the differential pressure.

In an embodiment, the method further comprises, measuring a first pressure of a suction side of the compressor; and when the first pressure is below a setpoint, adding liquid refrigerant to the separator.

In an embodiment, the method further comprises, measuring a second pressure of the top of the separator; and when the second pressure is above a setpoint, stop adding liquid refrigerant to the separator.

In an embodiment, the method further comprises, after the oil has been drained, boiling off any remaining refrigerant.

In an embodiment, the method further comprises, after elapse of the predetermined time, equalizing a pressure difference between a first pressure of a compressor suction side and a second pressure of the separator/evaporator.

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a refrigeration system in accordance with one embodiment;

FIG. 2 is a diagrammatical illustration of a separator vessel in accordance with one embodiment;

FIG. 3 is a schematic illustration of a refrigeration system control unit in accordance with one embodiment;

FIG. 4 is a flow diagram of a control method for a refrigeration system in accordance with one embodiment;

FIG. 5 is a schematic illustration of one embodiment of an oil batch return system;

FIG. 6 is a flow diagram of a control method for the oil batch return system; and

FIG. 7 is a schematic illustration of a control unit having stored thereon a batch oil return sequence and a refrigeration sequence.

DETAILED DESCRIPTION

U.S. Pat. No. 5,857,347 and WO 2023/198787, incorporated expressly by reference herein for all purposes, describes a refrigeration system with an evaporator and separator. A control unit ensures that the evaporator is provided with overfeed, which provides a greater rate of liquid refrigerant to the evaporator than the rate that actually evaporates.

WO 2023/198787 describes an improvement that can lower the refrigerant level in the separator up to 55% compared to the '347 patent. The refrigerant charge for a specific refrigeration system (separator, pipes and evaporator(s)) is determined in the design of the refrigeration system. In the present disclosure, the required level to keep the designed refrigerant charge in the separator and evaporator is calculated according to a mathematical algorithm based on measured inputs from the process. At low refrigeration capacities the required level is lower and at higher capacities the refrigerant level is higher in the separator. However, the actual refrigerant charge within the separator and evaporator is the same for both low capacity and high capacity.

The system of the '347 patent maintains a constant level in the separator, whereas in one embodiment, the control unit in the present disclosure maintains a constant refrigerant charge within the evaporator and separator.

In one embodiment, advantages of the present disclosure can include a more even control of liquid feed. The gain is lower wear on valves, compressors, and a more even load on the refrigeration plant.

With the present disclosure, it is possible to have a higher operating level in the separator vessel at full capacity compared to the '347 patent.

In one embodiment, a higher operating level itself may not increase the capacity of the evaporator (if all other parameters are unchanged). To increase capacity you need to add more heat by, for example, increasing food product flow or increasing food product infeed temperature to the freezer. However, an increased operating level increases the possibility to operate at a higher capacity.

In one embodiment, a feature of the present disclosure is to keep the same refrigerant charge in the evaporator/separator during base load and full capacity.

In one embodiment, the refrigerant level in the separator vessel is low at low capacity and higher at high capacities.

In one embodiment, a control unit is configured to keep the refrigerant charge in the evaporator, separator, and connecting pipes constant during operating conditions, meaning the refrigerant amount is constant.

The refrigeration system illustrated in FIG. 1 comprises a compressor 1, a condenser 2, a receiver 3, and an evaporator 4, each having at least one inlet and an outlet. It should be understood that FIG. 1 is a simplified diagram of the refrigeration system that includes the features of the present disclosure, and omits items such as auxiliary equipment, valves, instrumentation, and the like that are common on refrigeration systems. The refrigeration system further comprises a separator 5 having an inlet 6 and a first and a second outlet 7 and 8, respectively.

The first bottom outlet 7 of the separator 5 is connected to the inlet 9 of the evaporator 4. The outlet 10 of the evaporator 4 is connected to the middle inlet 6 of the separator 5. The second top outlet 8 of the separator 5 is connected to the inlet 11 of the compressor 1. The outlet 12 of the compressor 1 is connected to the inlet 13 of the condenser 2. The outlet 14 of the condenser 2 is connected to the inlet 15 of the receiver 3. Finally, the outlet 16 of the receiver 3 is connected to the inlet 6 of the separator 5 via a pipe 17 connecting the outlet 10 of the evaporator 4 with the inlet 6 of the separator 5. In one embodiment, the outlet 16 of the receiver 3 is connected to the separator 5 via a pipe 50 that does not mix with the evaporator 4 outlet.

In one embodiment, the separator 5 is positioned in a space which is cooled by the evaporator 4. This eliminates the need for insulating the separator 5. In one embodiment, the separator is positioned substantially laterally of the evaporator. This allows the separator 5 to feed the evaporator liquid refrigerant based on the head pressure on the refrigerant in the separator 5.

FIG. 2 is an illustration of the separator 5. In one embodiment, the separator 5 comprises a container 19 formed as a substantially cylindrical shell 20 with rounded end caps 21 and 22. The separator 5 has a first pipe forming the inlet 6 at a midsection, a second pipe forming the first outlet 7 in the bottom end cap 21, and a third pipe forming the second outlet 8 in the top end cap 22.

As shown from FIG. 1, in one embodiment of the separator 5, the first inlet pipe 6 may be connected via pipe 17 to the outlet 10 of the evaporator 4 so as to receive the mixture of liquid and vapor refrigerant therefrom. Optionally, in one embodiment, the outlet 10 of the evaporator 6 is connected to the inlet pipe 6, and the refrigerant from the receiver 3 enters the separator 5 through a separate pipe 50.

Further, in one embodiment, the inlet pipe 6 is directed tangentially into the container 19 such that the incoming mixture of liquid and vapor refrigerant will follow helical paths. Inside the cylindrical inner wall of the container 19, a foraminous partition 23 is provided, preferably a metallic net having a plurality of holes, openings, or perforations. This foraminous partition 23 has a smaller width or diameter than the shell of the container 19 such that there is a small gap between the partition 23 and the inner surface of the container 19.

In operation, the mixture of the vapor and liquid components of the refrigerant received from the evaporator 4 is ejected into the separator 5 towards the inner side of the foraminous partition 23. The liquid component follows a spiral or helical path penetrating the foraminous partition 23. It then flows downwards in the gap between the inner surface of the container 19 and the foraminous partition 23. The vapor component of the refrigerant does not penetrate the foraminous partition 23 but forms a helical flow upwards in the container 19 and will be evacuated through the top outlet pipe. Hereby, a most efficient separation of the vapor and liquid components of the refrigerant outputted from the evaporator is possible.

Above the opening of the inlet pipe a splash shield 24 is mounted so as to prevent liquid drops from moving upwards instead of downwards in the separator 5.

Above the bottom outlet 7 of the container 19 and below the desired level of the liquid refrigerant therein, a vortex limiter 25 is provided so as to reduce the risk of introducing vapor refrigerant into the liquid refrigerant in the lower section of the container 19.

In embodiments, the refrigerant can include ammonia, propane, carbon dioxide, a chlorofluorocarbon, a hydrochlorofluorocarbon, or a hydrofluorocarbon.

In operation, the mixture of liquid and vapor refrigerant from the evaporator 4 is thrown against the partition 23 with a certain minimum speed that gives the necessary centrifugal force to ensure the desired separation. The size of the openings in the partition 23, the viscosity of the liquid refrigerant, and the distance between the partition 23 and the inner surface of the container 19 are other design criteria that influence the efficiency of the separation.

The result is that the liquid component of the refrigerant is dropping down in the gap between the inner surface of the container 19 and the partition 23 while the vapor component of the refrigerant will flow helically upwards through the center of the container 19. Any droplets entrained by this helical flow will be thrown by centrifugal force out towards that part of the partition 23 that is positioned above the inlet 6 to the separator 5 and thus be trapped by the partition 23 so as to flow down in the gap between the partition 23 and the inner surface of the container 19.

It should be noted that the feeding in of fresh refrigerant into the separator 5 is via the end of the pipe 29 opening within the pipe 17 towards the inlet 6 of the separator 5. Thereby, any vapor component of the fresh refrigerant will be separated in the same way as the vapor component of the mixture returned from the evaporator 4. The fresh refrigerant also helps the circulation between the evaporator 4 and the separator 5. Optionally, in one embodiment, the outlet 10 of the evaporator 6 is connected to the inlet pipe 6, and the refrigerant from the receiver 3 enters the separator 5 through a separate pipe 50.

The vortex limiter 25, preferably having the form of a mesh cross, reduces vortex movement of incoming circulating liquid refrigerant and thereby simplifies the control of the level of the liquid refrigerant in the separator 5. Further, a vortex can be avoided at the bottom of the separator in order to ensure an even feed of liquid refrigerant to the evaporator, since a vortex could reduce the driving force and in extreme situations jeopardize the function of the evaporator.

In the present disclosure, the refrigeration system can include one or more analog level sensers 27. An analog sensor can provide continuous and instantaneous measurements of the refrigerant level in the separator 5. In one embodiment, the analog sensor 27 can be placed on a bypass line on the side wall of the separator 5.

In one embodiment, the refrigeration system may also include instruments to measure the conditions relating to the evaporator 4 and separator 5. Temperature sensor 31 measures the temperature of the medium being cooled after the evaporator 4. Temperature sensor 33 measures the temperature of the medium being cooled before the evaporator 4. Temperature sensor 32 can measure the temperature of the refrigerant from the separator 5. Temperature sensor 32 can be positioned on the evaporator 4 itself, on the outlet or return pipe therefrom or within the evaporator 4 below the liquid level therein.

Instrument 34 can measure the flow of the air passing over the evaporator 4. Instrument 34 can indirectly measure the air flow by determining the fan speed. Instrument 35 can measure the refrigerant flow out of the separator 5. Instrument 36 can measure the pressure of the refrigerant in the evaporator 4 or separator 5.

The refrigeration system also comprises a control unit 26 illustrated in detail in FIG. 3. The control unit 26 is connected to the instruments 31-36. Instruments 31-36 send measurements to the control unit 26, which are then processed by the control unit 26. The control unit 26 provides an output signal to the expansion valve 28 based on the received measurements.

The above described embodiment may be modified in several ways.

As an example, the outlet of the condenser 2 and receiver 3 could be connected directly to the separator 5 via a further, separate inlet positioned above the liquid refrigerant level therein. The outlet of the condenser 2 and receiver 3 could even be connected into the pipe leading from the first outlet of the separator 5 to the inlet of the evaporator 4.

In FIG. 1, the condenser 2 and receiver 3 constitutes a one-stage refrigeration system. However, a two-stage refrigeration system may also be used. Further, the condenser 2 and receiver 3 may comprise a closed economizer or an open economizer. Thus, the structure of the compressor 1 as well as the condenser 2 and receiver 3 may be varied within the scope of the invention.

Also, the evaporator 4 may take several forms and be used for cooling different fluids, such as a gas, e.g. air, as well as a liquid. The cooled fluid may be used for freezing, e.g. in a food freezing plant, but also for cooling, e.g. in an air conditioning system.

FIG. 3 is one embodiment of the control unit 26. The control unit 26 can be implemented in one or more computing devices.

A computing device includes at least one processor and a system memory connected by a communication bus. Depending on the exact configuration and type of device, the system memory may be volatile or nonvolatile memory, such as read only memory (“ROM”), random access memory (“RAM”), EEPROM, flash memory, a hard drive, solid state drive, CD ROM, DVD, or other disk storage, magnetic cassettes, magnetic tape, magnetic disk storage, and/or the like.

Those of ordinary skill in the art and others will recognize that system memory typically stores data and/or program modules that are immediately accessible to and/or currently being operated on by the processor. In this regard, the processor may serve as a computational center of the computing device by supporting the execution of instructions.

In this disclosure “engine” refers to logic embodied in hardware, circuitry, or software instructions, which can be written in a programming language, such as C, C++, COBOL, JAVA™, PHP, Perl, HTML, CSS, Javascript, VBScript, ASPX, Microsoft .NET™, Go, and/or the like. An engine may be compiled into executable programs or written in interpreted programming languages. Software engines may be callable from other engines or from themselves. Generally, the engines described herein refer to logical modules that can be merged with other engines, or can be divided into sub-engines. The engines can be stored in any type of computer-readable medium or computer storage device and be stored on and executed by one or more general purpose computers, thus creating a special purpose computer configured to provide the engine or the functionality thereof.

The control unit 26 includes a refrigeration load engine 40. The refrigeration load engine 40 is used for calculating an estimate of the heat load being applied to the evaporator 4. As shown, the refrigeration load engine 40 can use one or more inputs from instruments 31 to 36 for calculating refrigeration load. The refrigeration load engine 40 can rely on at least one instrument to calculate an estimate of the instantaneous refrigeration load on the evaporator 4. The instruments can provide, for example, measured variables including air temperature before and after the evaporator, air flow over the evaporator, refrigerant gas flowing out of the separator, refrigerant pressure in the evaporator, or any combination of variables. In one embodiment, the refrigeration load is calculated repetitively during the operation of the refrigeration system, so as to provide continuous estimate of the refrigeration load as the heat load on the evaporator 4 can be continuously changing over time. The refrigeration load can be calculated in units of BTU/hr or Watts (W), and the like.

The control unit 26 includes a level target engine 42. In one embodiment, the level target engine 42 uses the calculated refrigeration load to determine a level target of refrigerant in the separator 5.

In one embodiment, a level target is based on a linear function. For example, a linear function can be described by a plot of minimum to maximum designed refrigeration load versus the minimum to maximum designed refrigerant level. The function can be stored as a two-dimensional Table having the level target values correlated to the refrigeration load values.

In one embodiment, the level target is calculated to maintain a substantially constant refrigerant charge in the combined volumes of the evaporator/separator. Such calculation can take into consideration, for example, thermodynamic equations relating to the amounts of refrigerant in the system, refrigerant density, the heats of capacity, the heats of vaporization, and other thermodynamic quantities.

The control unit 26 may include a level control engine 44. The level control engine 44 uses the level target supplied by the level target engine 42. The level control engine 44 continuously receives a measured level of refrigerant in the separator 5 from an instrument, such as the analog level sensor 27. The analog level sensor can be placed on a bypass line of the separator 5. The level control engine 44 compares the level target from the level target engine 42 with the measured level and processes the difference into a signal that can be used to control the expansion valve 28. For example, when the measured level is below the level target, the level control engine 44 sends a signal to open the expansion valve 28 and increase the flow of refrigerant into the separator 5. When the measured level is above the level target, the level control engine 44 sends a signal to close the expansion valve 28 to reduce the flow of refrigerant into the separator 5. The strength of the signal that determines the position of the expansion valve 28 will vary based on the difference, the controller gain, and other factors.

In one embodiment, the level control engine 44 has an override function that prevents the level of refrigerant from exceeding a maximum permissible high level limit or falling below a minimum permissible low level limit in the separator 5. The permissible high and low level limits can be used to prevent equipment damage or prevent unsafe operating conditions. In the case of a high level limit being reached or exceeded, the level control engine 44 can fully close the expansion valve, for example, until such time as the level of refrigerant in the separator falls to safe operating level. In the case of a reaching or falling below a low level limit, the level control engine 44 can fully open the expansion valve, for example, until such time as the level of refrigerant in the separator reaches a safe operating level.

Additionally or alternatively, a suction valve 38 can be placed on the second outlet of the separator 5 that leads to the suction side of the compressor 1. In one embodiment, the level control engine 44 will close the suction valve 38 when a high level of refrigerant or a high rate of level change of refrigerant is detected in the separator 5.

Referring to FIG. 4, one embodiment of a method 50 of controlling the flow of refrigerant in a refrigeration system having at least one evaporator and one separator is illustrated.

In block 52, the separator and evaporator are charged with refrigerant based on the design criteria. In one embodiment, the refrigeration system is operated so as to maintain the amount of refrigerant charge within the separator and evaporator substantially constant. “Substantially constant” when used in reference to the amount of refrigerant charge can mean no more than 50% deviation, or 45% deviation, or 40% deviation, or 35% deviation, or 30% deviation, or 25% deviation, or 20% deviation, or 15% deviation, or 10% deviation, or 5% deviation. During charging, the evaporator 4 and separator 5 will simultaneously experience a heat load.

In block 58, the refrigeration load on the evaporator is continuously calculated by the control unit 26. Here, the refrigeration load can be calculated every second, or a fraction of a second, or may even perform a calculation after tens of seconds or minutes. Continuous calculation of the refrigeration load can also happen instantaneously or in real time as conditions change. The aim is to repetitively calculate the refrigeration load to provide the ideal target level in the separator 5.

In block 60, the level target is calculated by the control unit 26 based on the refrigeration load. In one embodiment, there are several functions to calculate the level target, including a function based on a linear relationship between the minimum to maximum refrigeration load and the minimum to maximum separator level. In one embodiment, the level target is based on a function that determines a level target that is calculated to maintain a substantially constant refrigerant charge within the evaporator 4/separator 5 system.

In block 62, the instantaneous or actual level in the separator 5 is measured via the analog sensor 27.

In block 64, the measured level is compared to the calculated level target generated in block 60 by the level target engine 42.

Block 66 is a decision block that determines whether the measured level has exceeded a high level limit or fallen below a low level limit in the separator 5. Block 68 is entered if either a high level or low level limit is exceeded. In block 68, the control unit 26 takes one or more corrective actions to correct the level in the separator when either a high level limit or low level limit is exceeded. Specifically, the level control engine 44 is configured to control the expansion valve 28 or the suction valve 38 as described in association with FIG. 3 above.

Block 70 is entered when neither a high level nor low level limit is exceeded in the separator 5. In block 70, the control unit 26 sends a signal to the expansion valve 28 depending on whether the measured level is less than or greater than the level target. The level target is calculated by the level target engine 42 based on a number or predetermined functions described in association with FIG. 3 above.

After block 70, the method returns to block 58 to repetitively calculate the refrigeration load and set a new target level if necessary.

The method of operating the evaporator/separator can have several advantages. The liquid feed can be controlled to be even and proportional to the refrigeration load.

The analog sensor makes it possible for the same separator to be used for several different load conditions and evaporators. This saves design time and adds flexibility. Instead of an analog sensor, a number of digital level switches can also be used to measure the level but it is preferred to use an analog sensor.

The system operates to raise the refrigerant level at high capacity and lower the refrigerant level at low capacity. This will allow a wider range of operating capacities, and in particular higher operating capacities for a specified evaporator/separator assembly. The refrigerant charge will be significantly smaller compared to a system that is controlled at a fixed level.

In one embodiment, the opening degree of the refrigerant from the liquid feed can be proportionally controlled to the actual need to provide an even liquid feed rate proportional to the refrigeration capacity. Even feed conditions are beneficial for the refrigeration plant operation to reduce wear and increase efficiency.

This disclosure also relates to an oil batch return (OBR) system and control sequence. The oil batch return sequence (“OBR sequence”) can be included in the refrigeration sequence for refrigeration systems operating with separator 5. The control unit 26 includes hardware circuitry or software.

FIG. 7 schematically depicts the control unit 26 configurable with a processor or microprocessor programmable with processor-executable instructions stored on a non-transitory machine-readable medium 706. The oil batch return sequence 702 and the refrigeration sequence 704 can be programmed into the control unit 26 or can be programmed into separate controllers. The control unit 26 includes a software application configured to perform both sequences 702 and 704.

It is understood that the refrigeration sequence 704 is programmed into the control unit 26. However, the refrigeration sequence 704 does not need to be disclosed in detail for understanding the oil batch return sequence 702 disclosed herein. Every different refrigeration system can use a different refrigeration sequence. The batch oil return sequence 702 includes using signals to and from the refrigeration sequence 704 as described herein. The batch oil return sequence 702 is generally performed prior to starting freezer production as it can be the optimal time to do the batch oil return sequence 702. After the freezer is stopped for defrost/cleaning, there will be a time of “pump down” (part of the standard refrigeration sequence 704). This means that the refrigeration compressor 1 is running and the suction valve 38 is open and the liquid feed valve 28 is closed. The air circulation fans in the freezer are running producing a heat load on the evaporator 4 which boils off the liquid refrigerant, such as carbon dioxide in evaporator 4 and the separator 5 but leaving the oil. After pump down there will normally be a small amount of oil mixed with a small amount of refrigerant at the bottom of the separator 5. There can also be oil left on inside surfaces generally in the separator 5 and evaporator 4 and the more time allowed after pump down, for example, during cleaning/drying/standstill, the more the oil has time to flow down to the drain point and can be returned via running the oil batch return sequence 702.

FIG. 5 is a schematic illustration of one embodiment of an oil batch return system. The compressor 1, separator 5, evaporator 4 as described with respect to FIG. 1 are illustrated, the other components are understood to be included, but have not been illustrated.

Referring to FIG. 5, a line 602, which can include piping, tubing, or the like, connects to a low point in the separator 5 that leads to an oil collection vessel 604. The oil is drained using a differential pressure to overcome any height difference between the low point and the oil destination.

Control drain valve 128 is provided on line 602. Drain valve 128 is controlled for draining oil. The separator 5 includes a suction line 8 (as shown in FIG. 1). A main suction valve 38 is provided on line 8. Line 8 is connected to line 602 via line 606. In one embodiment, the oil can be drained into the suction line 8 downstream from valve 38. A line 614 can also connect line 602 into vessel 604. In one embodiment, the oil can be drained into the vessel 604, which is then connected to the suction line 8 downstream from valve 38, i.e., on the compressor 1 side.

Liquid feed line includes the liquid feed or expansion valve 28, also described in connection with FIG. 1.

A pressure sensor 608 and 610 are provided on both sides of the valve 38. Pressure sensor 608 measures the pressure on the suction side of the compressor 1, and pressure sensor 610 measures the pressure of the vapor space in the separator 5 and evaporator. A connection between line 8 and line 602 occurs downstream of valve 38 (compressor side) and the pressure sensors 608 and 610 allow a differential pressure to be controlled between lines 8 and 602 to allow draining. Line 8 terminates into a knockout drum 612. Then, a further line connects the suction side of compressor 1 to the knockout drum 612.

The control unit 26 can be used to control the oil batch return sequence 702. In which case, the control unit 26 includes instructions for performing the oil batch return sequence 702 of FIG. 6 as well as the refrigeration sequence 704 for the refrigeration system. Each step is described more fully below.

Referring to FIG. 6, a flow diagram of the oil batch return sequence 702 is illustrated depicting a process of controlling the oil batch return system. The purpose of the oil batch return system is to drain the accumulated oil in the separator 5 and evaporator 4 back to any suitable vessel or to the compressor 1 suction. The oil batch return sequence 702 instructions are stored in memory 706 of the control unit 26 (FIG. 7). The control unit 26 can store both the instructions for the refrigeration sequence 704 and the oil batch return sequence 702. The instructions use signals, and the signals are depicted in FIG. 6 as bold text. Signals are used as the input and output of the oil batch return sequence 702. Signals may also be communicated between the oil batch return sequence 702 and the refrigeration sequence 704. The refrigeration sequence 704 can generate signals relating to the condition or operation relating to the refrigeration system and plant. A bold italic text in FIG. 6 denotes a setting or setpoint. A pentagon shape denotes a reference to other functions or refrigeration start-up functions.

The control unit 26 includes a human machine interface to interact with the control unit 26. The human machine interface allows a user to make decisions with respect to the operation of the refrigeration system and the oil batch return system, such as starting or stopping operations, receiving measurements, such as pressure and temperature, inputting control signals required by the system to operate, making adjustments to temperature and pressure setpoints, and the like.

The oil batch return sequence 702 is in inactive status (block 808) when either a production mode inactive signal (block 802) is false OR a signal indicating any appropriate freezer temperature exceeded the oil batch reset temperature set point (block 804) is false. The freezer temperature is based on, for example, an air or rail temperature rise above +7° C. (set point). The production mode inactive signal can mean that the freezer is not running in production (freezing food products), but is instead in, for example, defrost, cleaning, drying, standstill, etc. An example of a production mode inactive signal can be the opposite of signal (block 810) “Production mode active” signal. If defrost/cleaning is then selected in the refrigeration sequence 704, a pump down step and defrost step start and when the setpoint temperature in the freezer is reached (block 804), the oil batch return sequence 702 will enter stand by status (block 806). The next time the freezer is started for production, the production mode active signal (block 810) is triggered. If there is an oil batch return signal (block 812) granting permission from refrigeration plant, the oil batch return sequence 702 enters active status (block 814) signalling the oil return sequence 702 can commence.

The oil batch return sequence 702 enters standby (block 806) from inactive (block 808) when both the production mode inactive signal is true AND the freezer temperature exceeded the oil batch reset temperature set point signal (block 804) is true.

The oil batch return sequence 702 enters oil batch return active (block 814) when the production mode active signal (block 810) is true AND the oil batch return allowed signal from refrigeration plant (block 812) is true.

If the production mode active signal (block 810) is false, the oil batch return sequence 702 returns to oil batch return standby (block 806). If the oil batch return allowed signal from refrigeration plant (block 812) is false, the oil batch return sequence 702 returns to inactive status (block 808).

In the oil batch return sequence active status (block 814), the sequence 702 energizes/opens oil batch return drain valve 128 (block 818) on the condition that the batch drain differential pressure (BDDP) is greater than a pressure setpoint signal (block 816) is true. The purpose of opening the drain valve 128 is to drain the separator 5 and evaporator 4 of oil. The drain valve 128 will remain open for a predetermined oil batch return time period provided certain conditions are met. The batch drain differential pressure setpoint can be about 5 bar. The batch drain differential pressure can be calculated using the sensor 610 pressure minus the sensor 608 pressure (FIG. 5). When the batch drain differential pressure is not greater than the setpoint signal is false, meaning there is not enough pressure difference to push the oil back, then the oil batch return sequence 702 sends a request refrigeration signal (block 820).

In block 820, the purpose of “Request refrigeration” is to lower the compressor side pressure and increase the differential pressure to the batch drain differential pressure setpoint, to be able to push back the oil to the refrigeration plant. When the batch drain differential pressure (batch drain differential pressure) becomes greater than the pressure setpoint signal, the drain valve 128 opens. The drain valve 128 remains opens for the oil batch return time, as long as no other condition requires the drain valve 128 to close. The drain valve 128 opens and the timer starts the first time the batch drain differential pressure is higher than the setpoint. The drain valve 128 then remains open and the timer keeps running until the oil batch return timer reaches its setpoint, as long as the oil batch return sequence 702 is not terminated by loss of the oil batch return allowed signal from the refrigeration plant or by someone deselecting production mode from the control unit 26. When the oil batch return sequence 702 is finished (oil batch return time elapsed), the drain valve 128 closes and the refrigeration sequence 704 will move forward to prepare the freezer to begin production.

Request refrigeration signal (block 820) indicates the oil batch return sequence 702 begins to control the batch drain differential pressure at the setpoint, such as to 5 bar (batch drain differential pressure set point). The suction valve 38 is closed until the batch drain differential pressure set point is exceeded. If there is not enough pressure difference to push the oil back, refrigeration is requested, which means that the compressor 1 starts to lower the pressure on the compressor side (sensor 608). When this pressure is low enough that the pressure difference (610 minus 608) reaches the batch drain differential pressure setpoint the suction valve 38 starts to open to maintain this pressure difference as the compressor side pressure continues to decrease. The reason for this is to keep the compressor running smoothly. If the suction valve 38 did not open, the compressor side pressure (sensor 608) would go too low and the compressor would trip on low pressure.

From block 818, the oil batch return sequence 702 checks for the oil batch return time elapsed signal (block 822). Signal (block 822) is true when the time of the drain valve 128 being open is greater than a predetermined time setpoint. When the oil batch return time is elapsed, the drain valve 128 closes. When the signal (block 822) is true, the sequence enters block 824.

In block 824, after elapse of the oil batch return time signal (block 822), the sequence 702 performs equalizing of the oil batch return system. Equalizing is a procedure that is used when starting a freezer for production after defrost/cleaning. The purpose is to equalize the pressure difference between separator/evaporator pressure (sensor 610) and compressor side (sensor 608) by opening the suction valve 38 in a controlled way. If the suction valve 38 was just opened momentarily there is a risk for high load on the compressor 1 which could cause it to trip. The equalizing function looks at the differential pressure (610 minus 608), starts opening the suction valve 38 slowly, then opens more and more as the differential pressure decreases. When the differential pressure is close to zero (according to setpoint) the equalizing is completed and the suction valve 38 opens fully. The following take place during equalizing: the refrigeration request signal remains active AND liquid feed to separator 5 is disabled AND suction valve 38 is controlled to standard settings and function according to the refrigeration sequence.

When equalizing is completed, the signal for request refrigeration (block 832) is active, and the refrigeration sequence 704 is entered. Request refrigeration signal is a request to the refrigeration plant to get ready to provide refrigeration, i.e. start the compressor to lower the suction pressure to the setpoint corresponding to the evaporation temperature needed for production and get ready to provide liquid refrigerant to separator 5.

When the batch drain differential pressure is less than the setpoint, a request refrigeration signal (block 820 or 832) is true, the oil batch return sequence 702 sends a request refrigeration signal 820.

When the request refrigeration signal becomes true AND the oil batch return sequence 702 is active (block 814), the sequence 702 enters block 826 to raise the batch drain differential pressure. In block 826, the compressor 1 is starts to run. In block 826, the suction valve 38 is controlled to maintain the batch drain differential pressure at set point (about 5 bar) AND liquid feed to separator 5 is disabled. Block 826 can only be entered when the oil batch return sequence 702 is active, i.e. block 814.

A request refrigeration signal (block 832) is also triggered from block 824 after the oil batch return sequence 702 is finished (inactive), and the freezer is started for normal production. When entering block 826 the refrigeration request may already be active from block 820 (since if once activated in the oil batch return sequence 702 it stays active for the full duration).

When in block 826 and signal (block 828) is true, the sequence enters block 830. Signal (block 828) indicates when liquid refrigerant should be added to separator 5. The sequence 702 remains in block 826 as long as signal (block 828) remains false. Signal (block 828) is true allowing liquid refrigerant if the pressure from sensor 608 (FIG. 5) reaches or is lower than 10 bar AND NOT the pressure from sensor 610 has reached or exceeded 18 bar, which disables liquid refrigerant.

In block 830, the suction valve 38 is controlled to maintain the batch drain differential pressure set point (such as 5 bar) AND liquid feed is enabled to separator 5 AND override the minimum opening of control valve 38 for liquid feed. Block 830 is entered because there is not enough refrigerant, so running the compressor 1 will not help to lower the batch drain differential pressure. Instead, it would lower the pressure on both sensor 608 and 610 and not increase the differential pressure enough. By feeding liquid refrigerant via opening of valve 28, the liquid will expand (to gas/vapor phase) in the separator/evaporator and increase the pressure 610. Also, if no liquid was fed in this situation, the pressure 608 on the compressor suction side may go low and trip the compressor 1.

The oil batch return sequence 702 may include an “end of production” sequence. The end of production sequence includes a “pump down” sequence in which any remaining refrigerant is boiled off.

During cleaning and drying, oil remaining in the system is allowed to drain down to the low point (appendix) of the system. From this point oil can be drained and returned to an appropriate part of the refrigeration system.

During the oil batch return sequence 702, the main suction valve 38 can be used to maintain a differential pressure between the separator/evaporator and the refrigeration system suction side. This differential pressure is used to drive oil out from the low point and back towards the refrigeration system.

The oil return line 602 is connected to the refrigeration system at an appropriate point.

In one embodiment, the vessel 604 is installed at or above the compressor 1 to which the oil can be returned during start up. From the vessel 604, the oil may be fed back to the compressor 1 in a controlled way.

In one embodiment, an oil drain line 606 ends directly into the dry suction line, provided the refrigeration system is designed to handle this. A compressor protection vessel, such as vessel 612, of appropriate size, installed in the suction line prior to the compressor may capture the batch of oil and may then feed the oil in a controlled way to the compressor suction while the compressor is running.

The refrigeration sequence 704 can decide if the oil batch return sequence 702 is allowed to run or not by sending an oil batch return allowed signal 812. A relay allows the batch drain valve 128 provided the oil batch return allowed signal 812 is true. If the signal is lost, the oil batch return sequence 702 will end. For example, the refrigeration sequence 704 can end the oil batch return sequence 702 in case the oil receiver vessel 604 is filled up to a level that is considered as the maximum desired level. A high oil level in receiver 604 can be lowered before the oil batch return sequence 702 is allowed. Each refrigeration plant can decide the criteria for oil batch return sequence 702 allowed from refrigeration plant.

Embodiment, 1 includes refrigeration system, comprising:

• • a compressor 1, condenser 2, separator 5, and evaporator 4, wherein;
  • a condenser inlet is connected to an outlet of the compressor;
  • a separator inlet is connected to an outlet of the condenser;
  • an evaporator inlet is connected to a first outlet of the separator;
  • an evaporator outlet is connected to the separator inlet;
  • wherein the refrigeration system includes a refrigerant with an oil;
  • a control unit 26 having instructions stored thereon for executing an oil batch return sequence 702 comprising the following steps,
  • boiling the refrigerant within the evaporator 4 and the separator 5 that leaves the oil in the evaporator and the separator; and
  • draining the oil from the evaporator and/or separator.

Embodiment 2 includes the refrigeration system of embodiment 1, wherein the oil is drained to a compressor suction line 8 or to a vessel 604 connected to the compressor suction line.

Embodiment 3 includes the refrigeration system of embodiment 1, wherein the control unit 26 further has instructions stored thereon for executing the step of:

• • before draining the oil, checking a freezer temperature is above a predetermined setpoint.

Embodiment 4 includes the refrigeration system of embodiment 1, wherein the step of draining the oil includes opening a drain valve 128 while a timer counts down for a predetermined time, and closing the drain valve when the predetermined time expires.

Embodiment 5 includes refrigeration system of embodiment 4, wherein the control unit 26 further has instructions stored thereon for executing the step of:

• • keeping the drain valve 128 open for duration of the predetermined time as long as an oil batch return allowed signal from a refrigeration sequence 704 is true.

Embodiment 6 includes the refrigeration system of embodiment 4, wherein the drain valve 128 is placed on a line connecting a bottom of the separator 5 to a vessel 604 connected to a suction side of a compressor.

Embodiment 7 includes the refrigeration system of embodiment 1, wherein the control unit 26 further has instructions stored thereon for executing the step of:

• • draining the evaporator 4 and separator 5 of oil via providing a differential pressure (610 minus 608) between a refrigeration suction side and the separator and evaporator.

Embodiment 8 includes the refrigeration system of embodiment 7, wherein the differential pressure is measured across a control valve 38, the control valve is placed on a line connecting a top of the separator and a suction side of the compressor.

Embodiment 9 includes the refrigeration system of embodiment 8, wherein the control unit 26 further has instructions stored thereon for executing the steps of:

• • when the differential pressure is above a setpoint opening the control valve 38; and
  • when the differential pressure is below the setpoint closing the control valve.

Embodiment 10 includes the refrigeration system of embodiment 8, wherein the control unit 26 further has instructions stored thereon for executing the steps of:

• • when the differential pressure is below a setpoint, running the compressor 1 and the control valve 38 controls the differential pressure.

Embodiment 11 includes the refrigeration system of embodiment 10, wherein the control unit 26 further has instructions stored thereon for executing the steps of:

• • measuring a first pressure 608 of the suction side of the compressor 1; when the first pressure is below a setpoint, adding liquid refrigerant to the separator 5.

Embodiment 12 includes the refrigeration system of embodiment 11, wherein the control unit 26 further has instructions stored thereon for executing the steps of:

• • measuring a second pressure 608 of the top of the separator 5;
  • when the second pressure is above a setpoint, stop adding liquid refrigerant to the separator.

Embodiment 13 includes the refrigeration system of embodiment 1, wherein the control unit 26 further has instructions stored thereon for executing a refrigeration sequence 704 comprising the following steps:

• • sending a signal from the refrigeration sequence 704 to the oil batch return sequence 702 to allow the oil batch return sequence to run.

Embodiment 14 includes the refrigeration system of embodiment 13, wherein the control unit 26 further has instructions stored thereon for executing the following step:

• • sending a signal from the refrigeration sequence 704 to the oil batch return sequence 702 to stop the oil batch return sequence from running.

Embodiment 15 includes the refrigeration system of embodiment 13, wherein the control unit 26 further has instructions stored thereon for executing the following step:

• • sending a signal from the oil batch return sequence 702 to the refrigeration sequence 704 to request the refrigeration sequence to start a compressor 1 running.

Embodiment 16 includes the refrigeration system of embodiment 4, wherein the control unit 26 further has instructions stored thereon for executing the following step:

• • after the oil has been drained, boiling off any remaining liquid refrigerant.

Embodiment 17 includes the refrigeration system of embodiment 4, wherein the control unit 26 further has instructions stored thereon for executing the following step:

• • after elapse of the predetermined time, equalizing a pressure difference between a first pressure 610 of a compressor 1 suction side and a second pressure 608 of the separator 5/evaporator 4.

Embodiment 18 includes the refrigeration system of embodiment 17, wherein the control unit 26 further has instructions stored thereon for executing the following steps:

• • during equalizing, measuring the pressure difference and a control valve 38 positioned between the separator/evaporator and the compressor suction side is opened to lower the pressure difference until the pressure difference is close to zero.

Embodiment 19 includes a method of draining oil from a refrigeration system which includes,

• • a compressor 1, condenser 2, separator 5, and evaporator 4,
  • a condenser inlet is connected to an outlet of the compressor;
  • a separator inlet is connected to an outlet of the condenser;
  • an evaporator inlet is connected to a first outlet of the separator;
  • an evaporator outlet is connected to the separator inlet;
  • the separator and the evaporator using a refrigerant with an oil, the method comprising:
  • removing the refrigerant within the evaporator and the separator that leaves the oil in the evaporator and the separator; and
  • draining the oil from the evaporator/separator.

Embodiment 20 includes the method of embodiment 19, further comprising, before draining the oil, checking a freezer temperature is above a predetermined setpoint.

Embodiment 21 includes the method of embodiment 19, wherein the step of draining the oil includes opening a drain valve 128 while a timer counts down for a predetermined time, and closing the drain valve when the predetermined time expires.

Embodiment 22 includes the method of embodiment 21, further comprising,

• • keeping the drain valve 128 open for duration of the predetermined time as long as an oil batch return allowed signal 812 from a refrigeration sequence 704 is true.

Embodiment 23 includes the method of embodiment 19, further comprising,

• • draining the evaporator 4 and separator 5 of oil via providing a differential pressure between a refrigeration suction side and the separator and evaporator.

Embodiment 24 includes the method of embodiment 23, further comprising,

• • measuring the differential pressure;
  • when the differential pressure is above a setpoint opening a control valve 38; and
  • when the differential pressure is below the setpoint closing the control valve, wherein the control valve is placed on a line 8 connecting a top of the separator 5 and a suction side of the compressor 1.

Embodiment 25 includes the method of embodiment 24, further comprising,

• • when the differential pressure is below a setpoint, running the compressor 1 and the control valve 38 controls the differential pressure.

Embodiment 26 includes the method of embodiment 25, further comprising,

• • measuring a first pressure 608 of a suction side of the compressor 1; and
  • when the first pressure is below a setpoint, adding liquid refrigerant to the separator.

Embodiment 27 includes the method of embodiment 26, further comprising,

• • measuring a second pressure 610 of the top of the separator 5; and
  • when the second pressure is above a setpoint, stop adding liquid refrigerant to the separator.

Embodiment 28 includes the method of embodiment 21, further comprising,

• • after the oil has been drained, boiling off any remaining refrigerant.

Embodiment 19 includes the method of embodiment 21, further comprising,

• • after elapse of the predetermined time, equalizing a pressure difference between a first pressure 610 of the separator/evaporator and a second pressure 608 of a compressor 1 suction side.

EXAMPLE

The oil batch return sequence 702 is initiated when the freezer is started in production (freezing). The oil batch return sequence 702 is configured to hold or allow the “Request Refrigeration” signal as required to lower the batch drain differential pressure.

Enabling Start

When the freezer is NOT in production mode AND the air/rail temperature rise is above +7° C. (set point), the oil batch return sequence 702 is reset. The status changes from “Inactive” to “Stand By”.

Start of Sequence

Provided the oil batch return sequence 702 is in standby AND provided the oil batch return allowed signal from the refrigeration sequence 704 is received, the oil batch return sequence 702 will be activated when production is selected and started from the control unit HMI 26. A “production” instruction on the freezer control unit HMI 26 means the refrigeration sequence 704 is initiated to prepare the freezer for producing frozen products. In case the refrigeration sequence 704 also has the oil batch return sequence 702 programmed into it, the oil batch return sequence 702 will be run before the refrigeration sequence 704 (assuming that the oil batch return sequence 702 is in stand by).

Batch Drain Differential Pressure

The pressure difference between the evaporator/separator side and the refrigeration system (compressor 1 suction side) is a differential pressure calculated as sensor 610 pressure minus sensor 608 reading in bar. This differential pressure is referred to as actual batch drain differential pressure.

Hold Refrigeration Request

If the batch drain differential pressure remains at or above 5 bar (batch drain differential pressure set point) during the entire oil batch return time, the refrigeration request signal will not be activated.

Open the Drain Valve

When the oil batch return sequence 702 is active, the oil batch return drain valve 128 energizes/opens. The oil batch return timer starts to count down the oil batch return duration set point (150 sec). When the drain valve 128 opens, it allows the oil to drain to any suitable vessel in the refrigeration plant.

Request Refrigeration Activation

IF the actual batch drain differential pressure is lower than 5 bar (set point) THEN refrigeration request signal to the refrigeration plant is activated AND the suction valve differential pressure is controlled to 5 bar (batch drain differential pressure set point). (The suction valve 38 will remain closed until the batch drain differential pressure set point is exceeded).

The main suction valve 38 has a minimum opening setpoint. Override the minimum set point in the oil batch return sequence 702, meaning the liquid feed valve 28 may open in the oil batch return sequence 702 despite the suction valve 38 being closed or below the minimum set point.

The liquid feed valve 28 to the separator 5 is enabled when the actual pressure reading (item 608) at the compressor suction side of the suction valve (item 38) reaches or is lower than 10 bar (set point), which means that “allow liquid feed” signal in oil batch return sequence 702 is true, AND that at the same time the pressure on the evaporator/separator side (item 610) (set point) “Disable liquid feed on pressure” is NOT reached or exceeded. The signal for allow liquid feed in oil batch return sequence 702 is a signal that becomes true if pressure 608 is lower than the setpoint, such as 10 bar. Similarly the “disable liquid feed on pressure” is a signal that becomes true if pressure 610 is higher than a setpoint, such as 18 bar. If liquid feed is allowed to operate without a signal to “disable liquid feed on pressure,” when the freezer is warm (at startup), there is a risk that the load on the compressor becomes so high that the pressure cannot be kept below the maximum allowed working pressure and safety valves may blow. By disabling the liquid feed well before the pressure is close to maximum allowed this can be avoided.

If the Refrigeration Request has been activated in oil batch return sequence 702, it will remain active during the remaining of the sequence 702 and remain active when exiting.

Close the Drain Valve

IF/WHEN the oil batch return time (150 sec, set point) elapses OR the “oil batch return allowed” signal from the refrigeration system is lost, the oil and defrost drain valve 128 de-energizes and closes and the oil batch return sequence 702 ends.

Sequence End

The oil batch return sequence 702 ends with a simplified equalizing sequence. The refrigeration request will remain active AND the liquid feed will be disabled. The suction valve will be controlled to equalize the pressure according to settings.

When equalizing is completed the control unit enters the refrigeration main sequence.

Stop

Upon a manual or automatic stop request signal, the following actions are taken.

Oil batch return sequence 702 is terminated.

Drain valve 128 is de-energized/closed.

Power Failure

Upon detection of a power failure, the following actions are taken.

Oil batch return sequence 702 is terminated.

Drain valve 128 is de energized/closed.

Safety Reaction

Oil batch return allowed signal from refrigeration plant is lost. Drain valve 128 is de-energized/closed and oil batch return 702 sequence is terminated.

In one embodiment, the system provides a warning that the oil batch return sequence 702 was ended by the refrigeration system. In this warning, the oil batch return sequence 702 was ended by the refrigeration system. Oil may remain in the freezer.

In this disclosure, certain signals are used and have the following meanings.

“Oil batch return allowed” means a signal from refrigeration plant that allows oil batch return sequence 702 to run. If used, it may be connected to a high-level switch in a receiver for oil in the refrigeration plant. No signal/open contacts mean the refrigeration plant cannot receive oil return.

“Refrigeration Request” means a signal from oil batch return sequence 702 to the refrigeration sequence 704. The oil batch return sequence 702 requires an immediate confirmation reply from the refrigeration sequence 704.

In this disclosure, certain settings are used and have the following meanings.

“Oil batch return reset temperature” is any appropriate available temperature in freezer, rail temperature, air temperature, etc. The set point needs to be exceeded to reset the oil batch return sequence 702 to standby status.

“Batch drain differential pressure” set point indicates a differential pressure set point which is controlled by the sequence 702 (std =5 bar). The actual batch drain differential pressure is calculated as pressure readings from the difference of sensor 610 minus sensor 608.

“Allow liquid feed” indicates a pressure set point below which liquid feed to the separator/evaporator may be enabled if not prohibited by other values or settings. If sensor 608 is less than a set point AND NOT the pressure from sensor 610 reaches or exceeds a higher setpoint, then liquid feed is allowed in oil batch return sequence 702. If the separator/evaporator has only a very small amount of refrigerant (mixed with the oil) when the oil batch return sequence 702 starts it may not be enough to keep the pressure up in the separator/evaporator to achieve the 5 bar batch drain differential pressure. In that case, it will not help to only start the compressor. It would lower the pressure on both sensors 608 and 610 and not increase the differential pressure enough. By feeding liquid, the liquid will expand (to gas/vapor phase) in the separator/evaporator and increase the pressure 610. Also if no liquid was fed in this situation, the pressure on the compressor suction side 608 may go low and trip the compressor.

“Oil batch return time” indicates maximum allowed time for the oil batch return sequence in active mode. Examples include: Default set point is 150 sec., Hysteresis to initiate: 30 sec., and Hysteresis to end: 0 sec.

“Disable liquid feed on pressure” signal uses a set point from the refrigeration sequence 704. If the pressure at 610 (the separator/evaporator pressure) exceeds the set point “Disable liquid feed on pressure”, then the liquid feed is disabled. The setpoint for “disable liquid feed on pressure” is a part of the refrigeration sequence 704 (i.e., it is a setpoint of a freezer regardless whether there is an oil batch return system). The actual value for the setpoint is set well below the maximum allowed working pressure of the evaporator. In FIG. 6, as an example, the setpoint can be 18 bar, but it could be higher if the system is designed for higher working pressure.

While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

Claims

1. A refrigeration system, comprising:

a compressor, condenser, separator, and evaporator, wherein;

a condenser inlet is connected to an outlet of the compressor;

a separator inlet is connected to an outlet of the condenser;

an evaporator inlet is connected to a first outlet of the separator;

an evaporator outlet is connected to the separator inlet, wherein the separator is configured to receive vapor and liquid refrigerant from the evaporator outlet and from the condenser via an expansive valve;

wherein the refrigeration system includes a refrigerant with an oil;

a control unit having instructions stored thereon for executing an oil batch return sequence comprising the following steps: boiling the refrigerant within the evaporator and the separator that leaves the oil in the evaporator and the separator; and draining the oil from the evaporator and/or separator.

2. The refrigeration system of claim 1, wherein the oil is drained to a compressor suction line or to a vessel connected to the compressor suction line.

3. The refrigeration system of claim 1, wherein the control unit further has instructions stored thereon for executing the step of:

before draining the oil, checking that a freezer temperature is above a predetermined setpoint.

4. The refrigeration system of claim 1, wherein the step of draining the oil includes opening a drain valve while a timer counts down for a predetermined time, and closing the drain valve when the predetermined time expires.

5. The refrigeration system of claim 4, wherein the control unit further has instructions stored thereon for executing the step of:

keeping the drain valve open for duration of the predetermined time as long as an oil batch return allowed signal from a refrigeration sequence is true.

6. The refrigeration system of claim 4, wherein the drain valve is placed on a line connecting a bottom of the separator to a vessel connected to a suction side of a compressor.

7. The refrigeration system of claim 1, wherein the control unit further has instructions stored thereon for executing the step of:

draining the evaporator and separator of oil via providing a differential pressure between a refrigeration suction side and the separator and evaporator.

8. The refrigeration system of claim 7, wherein the differential pressure is measured across a control valve, the control valve is placed on a line connecting a top of the separator and a suction side of the compressor.

9. The refrigeration system of claim 8, wherein the control unit further has instructions stored thereon for executing the steps of:

when the differential pressure is above a setpoint opening the control valve; and

when the differential pressure is below the setpoint closing the control valve.

10. The refrigeration system of claim 8, wherein the control unit further has instructions stored thereon for executing the steps of:

when the differential pressure is below a setpoint, running the compressor and the control valve controls the differential pressure.

11. The refrigeration system of claim 10, wherein the control unit further has instructions stored thereon for executing the steps of:

measuring a first pressure of the suction side of the compressor;

when the first pressure is below a setpoint, adding liquid refrigerant to the separator;

measuring a second pressure of the top of the separator; and

when the second pressure is above a setpoint, stop adding liquid refrigerant to the separator.

12. (canceled)

13. The refrigeration system of claim 1, wherein the control unit further has instructions stored thereon for executing a refrigeration sequence comprising the following steps:

sending a signal from the refrigeration sequence to the oil batch return sequence to allow the oil batch return sequence to run;

sending a signal from the refrigeration sequence to the oil batch return sequence to stop the oil batch return sequence from running; and

sending a signal from the oil batch return sequence to the refrigeration sequence to request the refrigeration sequence to start a compressor running.

14-16. (canceled)

17. The refrigeration system of claim 4, wherein the control unit further has instructions stored thereon for executing the following step:

after elapse of the predetermined time, equalizing a pressure difference between a first pressure of a compressor suction side and a second pressure of the separator/evaporator; and

during equalizing, measuring the pressure difference and a control valve positioned between the separator/evaporator and the compressor suction side is opened to lower the pressure difference until the pressure difference is close to zero.

18. (canceled)

19. A method of draining oil from a refrigeration system which includes

a compressor, condenser, separator, and evaporator,

a condenser inlet is connected to an outlet of the compressor;

a separator inlet is connected to an outlet of the condenser;

an evaporator inlet is connected to a first outlet of the separator;

an evaporator outlet is connected to the separator inlet, and wherein the separator is configured to receive vapor and liquid refrigerant from the evaporator outlet and from the condenser via an expansive valve;

the separator and the evaporator using a refrigerant with an oil, the method comprising:

removing the refrigerant within the evaporator and the separator that leaves the oil in the evaporator and the separator; and

draining the oil from the evaporator/separator.

20. The method of claim 19, further comprising

before draining the oil, checking that a freezer temperature is above a predetermined setpoint.

21. The method of claim 19, wherein the step of draining the oil includes opening a drain valve while a timer counts down for a predetermined time, and closing the drain valve when the predetermined time expires.

22. The method of claim 21, further comprising

keeping the drain valve open for duration of the predetermined time as long as an oil batch return allowed signal from a refrigeration sequence is true.

23. The method of claim 19, further comprising

draining the evaporator and separator of oil via providing a differential pressure between a refrigeration suction side and the separator and evaporator.

24. The method of claim 23, further comprising

measuring the differential pressure;

when the differential pressure is above a setpoint opening a control valve; and

when the differential pressure is below the setpoint closing the control valve, wherein the control valve is placed on a line connecting a top of the separator and a suction side of the compressor.

25. The method of claim 24, further comprising

when the differential pressure is below a setpoint, running the compressor and the control valve controls the differential pressure.

26. The method of claim 25, further comprising

measuring a first pressure of a suction side of the compressor; and

when the first pressure is below a setpoint, adding liquid refrigerant to the separator;

measuring a second pressure of the top of the separator; and

when the second pressure is above a setpoint, stop adding liquid refrigerant to the separator.

27-28. (canceled)

29. The method of claim 21, further comprising

after elapse of the predetermined time, equalizing a pressure difference between a first pressure of the separator/evaporator and a second pressure of a compressor suction side.